Fluorescence Spectroscopy of AdamBODIPY Single Crystals

Interest in the fluorescence of organic solids is increasing with the development of nanosensors and research into new molecules with aggregation-induced fluorescence properties. We have gone beyond the qualitative observation of fluorescence by analyzing the luminescence properties of planar single crystals of a 4,4’-difluoro-4-bora-(3a,4a)-diaza-s-indacene (BODIPY) derivative with micrometric dimensions. A simple Frenkel exciton model applied to this crystal predicts one band. From time-resolved fluorescence spectra, three emissions were distinguished. The shortest-lived one at l = 547 nm was predicted by theory and corresponds to an exciton lifetime of 0.9 ns. A trap with an intermediate emission lifetime of 1.2 ns was found at 569 nm and a final trap at 620 nm has a lifetime of 1.9 ns. These attributions were confirmed by the study of the polarization of these emissions. The 547 nm emission was polarized along the long axis of the crystal as predicted by the Frenkel exciton model. The 569 nm emission was polarized perpendicularly to the plane of the crystal and the 620 nm emission was polarized along the short axis. Thus, the two red- shifted bands were related to well-defined defects with specific orientations in the crystal. Fluorescence lifetime imaging measurements showed that the density of these defects is not uniform and that under our synthesis conditions, they are formed in the initial steps of the growth and therefore appear in the center of the crystals